Wavelength-division multiplex (WDM) communication enables transmission paths to increase the transmission capacity thereof due to reduction in wavelength spacing and the number of optical signals input thereto. As the number of optical signals increases and the wavelength spacing reduces, optical phase distortion due to cross phase modulation occurring in the transmission paths increases. Thus, it is difficult to achieve both the increase in transmission capacity and an increase in transmission distance at the same time. When optical phase distortion occurring in the transmission paths is able to be compensated, optical signals with an increased transmission capacity are able to be transmitted over a long distance.
FIG. 12 illustrates optical phase distortion occurring in a transmission path. In FIG. 12, (a) illustrates an example of time and optical intensity characteristics of a wavelength-division multiplexing optical signal (WDM signal). Intensity variation of the WDM signal is some gigahertz (GHz), and a modulation component is tens of GHz. When the WDM signal illustrated in (b) of FIG. 12 (wavelength and optical intensity characteristics) is input to a transmission path D, a WDM signal similar to the WDM signal illustrated in (b) of FIG. 12 is output as illustrated in (c) of FIG. 12. Here, low-speed intensity variation (some GHz) exhibited by the WDM signal causes optical phase distortion in the transmission path D, and the WDM signal (of a receiving side) output from the transmission path D has, as illustrated in time and optical phase distortion characteristics in (d) of FIG. 12, an optical phase distortion component that varies over time.
There exists a related-art technique for compensating optical phase distortion. With this technique, optical phase distortion is compensated by bifurcating an optical signal degraded in a transmission path, inputting one of the bifurcated optical signal to a lithium niobate (LN; LiNbO3) phase modulator, and controlling the LN phase modulator in accordance with output of another photoreceiver. Optical phase distortion is able to be compensated at, for example, a transmitting end, a receiving end, and repeating points for the optical signal. Compensating devices are provided at a poststage of a transmitter at the transmitting end, at a prestage of a receiver at the receiving end, and at poststages of optical amplifiers of the repeaters at the repeating points.
As the related art, for example, Japanese Laid-open Patent Publication Nos. 07-58699 and 2007-189402 are disclosed.
With the related art technique, optical phase distortion occurring in a wide transmission band (for example, the C band) is not able to be compensated. With the related art, to address comparatively low-speed intensity variation of the above-described wavelength-division multiplexed optical signal, optical phase distortion due to cross phase modulation is compensated by phase modulation performed on the optical signal. In this case, the following problems arise.
1. The existing phase modulator causes a large insertion loss and has a large polarization dependence.
2. The amount of compensation is small when optical phase distortion occurring in a distributed manner in the transmission path is compensated by the phase modulator.
When optical phase distortion is compensated with the phase modulator, additional insertion loss occurs due to installation of the phase modulator. Furthermore, since the compensation is performed in terms of a lumped constant, phase distortion varying and occurring in a distributed manner is compensated partially. Furthermore, to adjust the modulation index, it is required that phases after the modulation with the phase modulator be monitored. Thus, the configuration becomes complex.
In view of the above description, it is desired that optical phase distortion occurring in a wide transmission band be able to be compensated with a simple configuration and small additional loss.